lang: Cant build with CPI enabled with empty context struct

[tomlinton]

E.g. cargo build-bpf --features cpi in tutorials/basic-0/programs/basic-0 leads to:

error[E0107]: this struct takes 0 lifetime arguments but 1 lifetime argument was supplied
  --> programs/basic-0/src/lib.rs:5:1
   |
5  | #[program]
   | ^^^^^^^^^^- help: remove these generics
   | |
   | expected 0 lifetime arguments
   |

#1598 is passing an account for no reason to avoid this.